In manufacturing electronic devices, plasma etching is performed on an etching target layer of a target object to be processed. A capacitively coupled parallel plate type plasma processing apparatus is known as a plasma processing apparatus for use in the plasma etching. The parallel plate type plasma processing apparatus includes a processing chamber, an upper electrode and a lower structure having a lower electrode. The target object is mounted on the lower structure so as to face a processing space in the processing chamber. In this plasma processing apparatus, a processing gas is supplied into the processing chamber and a high frequency power is applied to the upper electrode or the lower electrode. Thus, a plasma of the processing gas is generated in the processing space, and the target object, which is mounted on the lower structure so as to face the processing space, is etched by the plasma of the processing gas.
During the plasma etching process, an etching by-product generated due to the etching of the target object is formed as a deposit on surfaces defining an inner space of the processing chamber. The deposit may change the state in the plasma processing apparatus. Therefore, in the plasma processing apparatus, it is required to perform a cleaning for removing such deposit.
As a general cleaning method for removing a deposit generated in the plasma processing apparatus, there is provided a method of converting the deposit into a gaseous reaction by-product by causing chemical reaction on the deposit and exhausting the reaction by-product. The cleaning method using the chemical reaction may include a method of exhausting a reaction by-product by repeatedly performing oxidation reaction and reduction reaction on a deposit, e.g., a plasma polymer. Such method is disclosed in Japanese Patent Application Publication No. 1997-129623.
A semiconductor layer is generally used as the etching target layer to be subjected to the plasma etching. However, recently, the plasma etching is performed on a metal-containing layer. The metal-containing layer, i.e., a metal layer, may be a Cu layer, a PtMn layer or the like. The PtMn layer is used as a part of a magnetoresistive random access memory (MRAM) having a magnetic tunnel junction (MTJ) structure.
When the plasma etching is performed on the metal layer, e.g., the PtMn layer in the MRAM device, an etching by-product containing a metal is formed as a deposit on the surfaces defining the inner space of a processing chamber (hereinafter, referred to as “inner surfaces”). When the deposit is adhered to the inner surfaces, a plasma state in the processing chamber is changed and, thus, an etching rate of the plasma etching is changed. For example, in the parallel plate type plasma processing apparatus, a heavy metal contained in the etching by-product is moved toward an upper electrode by a sheath electric field generated near the target object. This metal is adhered to the upper electrode to form a deposit. The deposit adhered to the upper electrode generates a capacitance component, which leads to the change in impedance and the decrease in etching rate. Further, due to an etchant consumed by the deposit, the amount of the etchant supplied to the etching target layer becomes insufficient. As a result, the etching rate may further be decreased.
Therefore, it is necessary to remove the deposit containing the metal from the etching by-product (hereinafter, referred to as “metal deposit”) because it changes the plasma state in the processing chamber. However, it is generally difficult to generate a gaseous reaction by-product by causing chemical reaction on the metal contained in the deposit and exhaust the gaseous reaction by-product. In other words, the metal contained in the deposit is mostly in a solid state even after the chemical reaction occurs. Therefore, the metal deposit cannot be removed by the general cleaning method.
The cleaning method using oxidation reaction and reduction reaction can remove only a metal having high ionization tendency, i.e., a metal that is easily ionized, e.g., K, Ca, Na or the like. This is because such a method using the oxidation reaction and the reduction reaction removes a deposit by weakening a binding force of the deposit. Therefore, in such a method, it is difficult to remove a deposit containing a chemically stable metal, i.e., a metal having low ionization tendency, or it takes a long period of time to remove such deposit.
In the parallel plate type plasma processing apparatus, a space where a plasma is generated (hereinafter, referred to as “plasma space”) exists between the upper electrode and the lower electrode. Accordingly, although the metal deposit formed on the upper electrode can be removed from the upper electrode, a metal in the metal deposit is re-adhered to the lower electrode. This is because the heavy metal forming the deposit is rapidly moved toward the lower electrode opposite to the upper electrode without being removed from the plasma space by the sheath electric field generated between the plasma space and the electrodes. As a result, the metal is re-adhered to the lower structure or the target object mounted on the lower structure.
Therefore, there is required a technique for removing a deposit containing a metal having low ionization tendency, which is generated in a capacitively coupled parallel plate type plasma processing apparatus.